Ultrasound promoted Wurtz coupling of alkyl bromides and dibromides

Sonochemically enhanced Wurtz coupling using lithium metal has been investigated for a number of isomeric alkyl bromides under a variety conditions. The products result from direct coupling of short lived radicals formed at the metal surface rather than the secondary radicals which can be formed during coupling of aromatic halides and thus give rise to a single major product. Coupling has been extended to dibrominated aryl and alkyl compounds as well as showing that aryl–alkyl coupling is possible. Dibrominated alkyls were found to give low molecular weight oligomers although no reaction occurred for 1,2-isomers. The growth of oligomers in THF may be solubility limited. A simple model is proposed to explain these findings.

► Wurtz coupling using lithium metal of alkyl bromides.
► Single products formed from short lived radicals, unlike aromatic bromides.
► 1,2-Dibromides do not couple.
► 1,3- and 1,4-Dibromides couple to give oligomers.